patch 8.1.1857: cannot use modifier with multi-byte character

Problem:    Cannot use modifier with multi-byte character.
Solution:   Allow using a multi-byte character, although it doesn't work
            everywhere.

src/misc2.c

@@ -2819,10 +2819,10 @@ find_special_key(
 		    l = mb_ptr2len(bp + 1);
 		else
 		    l = 1;
-		/* Anything accepted, like <C-?>.
+		// Anything accepted, like <C-?>.
-		 * <C-"> or <M-"> are not special in strings as " is
+		// <C-"> or <M-"> are not special in strings as " is
-		 * the string delimiter. With a backslash it works: <M-\"> */
+		// the string delimiter. With a backslash it works: <M-\">
-		if (!(in_string && bp[1] == '"') && bp[2] == '>')
+		if (!(in_string && bp[1] == '"') && bp[l + 1] == '>')
 		    bp += l;
 		else if (in_string && bp[1] == '\\' && bp[2] == '"'
 							       && bp[3] == '>')

src/testdir/test_mapping.vim

@@ -239,6 +239,12 @@ func Test_map_meta_quotes()
   iunmap <M-">
 endfunc
 
+func Test_map_meta_multibyte()
+  imap <M-á> foo
+  call assert_equal('foo', maparg("\<M-á>", 'i'))
+  iunmap <M-á>
+endfunc
+
 func Test_abbr_after_line_join()
   new
   abbr foo bar
@@ -292,7 +298,7 @@ func Test_map_timeout_with_timer_interrupt()
   let g:val = 0
   nnoremap \12 :let g:val = 1<CR>
   nnoremap \123 :let g:val = 2<CR>
-  set timeout timeoutlen=1000
+  set timeout timeoutlen=100
 
   func ExitCb(job, status)
     let g:timer = timer_start(1, {-> feedkeys("3\<Esc>", 't')})

src/version.c

@@ -769,6 +769,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1857,
 /**/
     1856,
 /**/